desktop-shell: Keep windows on screen when moving
desktop-shell: Keep windows on screen when moving

We add a primitive constraining function to at least prevent users from
moving the window up under the panel.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=73421

krezovic/weston,giucam/weston,jonnylamb/weston,mchalupa/weston,Gnurou/weston,eyolfson/weston,sir-murray/weston,Fantu/compositor-spice,jonnylamb/weston,sir-murray/weston,eyolfson/weston,Fantu/compositor-spice,kwm81/weston,mchalupa/weston,Fantu/compositor-spice,giucam/weston,udoprog/weston,Gnurou/weston,xorgy/weston,Tarnyko/weston-xdg_surface_present,Tarnyko/weston-xdg_surface_present,krezovic/weston,udoprog/weston,kwm81/weston,xorgy/weston